Field of the Invention
The invention relates to a particle trap for separating particles out of a fluid or exhaust gas flow, having at least one flow channel through which the fluid flows. The invention also relates to a process for separating particles out of a fluid or exhaust gas flow. Additionally, the invention relates to a honeycomb body and an exhaust system having a particle trap.
A particle trap of the type described above is used to separate solid particles which are contained in a flowing fluid out of the flow. Particle traps of that type have particular practical significance when cleaning diesel exhaust gases from vehicles. In that case, it must be possible to regenerate the particle trap by oxidizing the particles. It should be possible to fit the trap in a pipe, such as for example in an exhaust system of a motor vehicle.
Particles in the exhaust gases of a motor vehicle often accumulate on a coating and/or an outer wall, such as for example in the exhaust system and/or in a catalytic converter of the motor vehicle. In the event of load changes, they are then expelled in the form of a cloud of particles, such as for example a cloud of soot.
In order to separate out the particles, it is customary to use screens, which are in some cases also referred to as filters. Although screens are fundamentally able to trap the particles, they have two significant drawbacks: firstly, they may become blocked, and secondly they cause an undesired pressure drop. If screens are not used, there is a risk that the honeycomb body of the catalytic converter will become blocked, i.e. the pores of the precious metals will become covered, and it will be impossible for either pollutants or oxygen to diffuse to the precious metals at the locations which have been covered by the particles. Moreover, it is necessary to observe statutory values for motor vehicle emissions, which would be exceeded without a reduction in the level of particles. It is therefore necessary to provide elements for trapping exhaust-gas particles which avoid the drawbacks of the screens and filters and do not impair the activity of the catalytic converters.
It is accordingly an object of the invention to provide a particle trap and a process for separating particles out of an exhaust gas flow and a honeycomb body and an exhaust system having a particle trap, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and which separate particles out of an exhaust gas flow with the highest possible efficiency, while the particles that have been separated out can also subsequently be removed by oxidation, so that the particle trap can be regenerated.
With the foregoing and other objects in view there is provided, in accordance with the invention, a particle trap for separating particles out of an exhaust gas flow. The particle trap comprises at least one flow channel through which an exhaust gas flows. The flow channel has at least one wall with regions. At least the regions are made from porous or highly porous material. The flow channel also has internals acting as a diverting or guiding structure for diverting the particles out of the exhaust gas flow to the regions made from porous or highly porous material.
With the objects of the invention in view, there is also provided a process for separating particles out of an exhaust gas flow, which comprises providing a particle trap with at least one flow channel having a porous or highly porous channel wall, and providing at least one diverting or guiding structure in the at least one flow channel. Particles of an exhaust gas flowing through the particle trap are thrown or diverted onto the porous channel wall by the at least one diverting or guiding structure.
The above-described structure and mode of operation of the invention are advantageous because it has surprisingly been seen that the particles which are diverted out of the flow or exhaust gas toward the regions made from porous or highly porous material by the internals remain attached to the porous channel wall as a result of interception and/or impaction. The pressure differences in the flow profile of the flowing fluid are of importance in bringing about this effect. The deflections allow additional local reduced pressure and increased pressure conditions to be established, which lead to a filtration effect through the porous wall, since the pressure differences have to be compensated for.
In contrast to closed screen or filter systems, the particle trap is open, since there are no blind flow alleys. These properties can also serve in this case to characterize the particle trap, for example by saying that an open view of 20% means that, when viewed in cross section, it is possible to see through approximately 20% of the area. This would correspond to an area of  greater than 0.1 mm2 for a 600 cpsi carrier with d (hydraulic)=0.8 mm.
Suitable materials for the particle trap are metal, plastic, ceramic, etc., at least for that part of the channel wall which lies approximately opposite the internals that are porous when constructing the flow channels.
The porous material being used may be any porous or highly porous sintered and/or fibrous substance. In addition to the deposition and adhesion behavior with respect to soot particles (for example large geometric surface area, large inflow area, suitable depth of the structure), the dimensional stability, strength and resistance to corrosion of the porous or highly porous material also play a role.
The particle trap according to the invention and the process according to the invention are particularly significant when used on honeycomb bodies which are known from catalytic converters of motor vehicles. In this case, a porous channel wall is produced, for example, by forming the entire honeycomb body from porous material. When forming the honeycomb body from porous or highly porous material, the extent to which fluid can flow through the porous material is of interest to the extent that it is possible to achieve a radial flow through the particle trap through the use of completely porous channel walls.
The channel wall is considered to be the channel wall over the entire length of the flow channel or a part of the entire channel wall.
Through the use of the diverting or guiding structure, that is to say the internals, particles are deflected out of the fluid, at certain velocities, onto the porous wall, to which they adhere. In the particle trap, the particles are separated out on the basis of the xe2x80x9cstrikerxe2x80x9d and xe2x80x9ccatcherxe2x80x9d principle. The diverting structure, as the xe2x80x9cstrikerxe2x80x9d, diverts the particles toward the xe2x80x9ccatcherxe2x80x9d of the porous channel wall or interlayer. The diverting structures within a flow channel may be identical or different, and the structuring of the flow channels within a particle trap may also vary.
The cross section of the flow channel is dimensioned in such a way that even the smallest point in the channel allows particles of an average size to pass through. The cross section is influenced by the diverting or guiding structures and may be identical or different over the length of the flow channel.
In accordance with another feature of the invention, the region made from porous or highly porous material is formed by an interlayer which is disposed on the nonporous channel wall. This provides a greater freedom of choice in terms of the materials being used.
In accordance with a further feature of the invention, the internals include at least one projection, hump, protuberance or the like, which is disposed so as to project out of the channel wall opposite the region made from porous or highly porous material and, if appropriate, in an axially offset manner. In this way, the internals can be produced easily in terms of manufacturing technology and at optimum locations.
In accordance with an added feature of the invention, a plurality of projections, humps, protuberances or the like are disposed in succession, and all project equally far from the channel wall into the interior of the flow channel, as seen in the flow direction of the fluid. Alternatively, a plurality of projections, humps, protuberances or the like are disposed in succession, projecting progressively further from the channel wall into the interior of the flow channel as seen in the flow direction. In both of these cases, it is also possible, according to an advantageous configuration, to provide a grouped configuration of the projections, humps, protuberances or the like, with regions also extending in the peripheral direction of the channel wall.
A suitable adaptation of the diverting and guiding structures and the structuring of the flow channels makes it possible, for example, to achieve a particular effect. According to that effect, within a particle trap, in successive partial regions, first of all the particles which are diverted by the structures with a first flow velocity are deflected onto the porous channel wall, and then the particles with a second, for example higher particle velocity, which have not been diverted, are diverted on the smaller diverting structures, etc. In this case, the particle trap offers a separating feature for any flow velocity, depending on the operating state of a vehicle.
It is also possible, through the use of the geometry of the diverting and guiding structures, for the flow velocity and the flow pressure of the fluid flowing through the particle trap to be manipulated and adapted to an optimum separation rate. In this way, by way of example, the increasing radial extent of the projections, humps, protuberances or the like into the interior of the flow channel ensures that not only are the particles which are at a higher velocity diverted, but also those particles which lie close to the porous channel wall are diverted. In this way, the diffusion distance which a particle has to cover through the flow to the wall region made from porous or highly porous material becomes ever shorter.
In accordance with an additional feature of the invention, the internals acting as a diverting or guiding structure each deflect between 2% and 15%, in particular between 4% and 8%, of the particles still contained in the exhaust gas toward the regions made from porous or highly porous material, which has proven particularly advantageous. Corresponding tests have shown that a particle trap constructed in this way causes only a very low pressure drop, while with a sufficient number of diverting or guiding structures disposed offset with respect to one another and/or one behind the other in the flow direction, very effective removal of particles is nevertheless ensured. If appropriate, it is also possible in this case for a plurality of particle traps of this type, each having a few diverting or guiding structures, to be connected in series. If appropriate, they are supported with respect to one another. The number of the diverting or guiding structures is to be selected in particular in such a way that at least a statistical diversion of the entire gas stream toward or through the porous material is effected. It is currently known from tests that if even 10 to 15 diverting structures of this type are connected one behind the other in the flow direction, a separation efficiency of over 90% and even over 95% of the particles contained in the flow is achieved.
In accordance with yet another feature of the invention, the diversion of the particles still contained in the exhaust gas is effected by suitably selecting the porosity of the porous or highly porous material and/or the size of the diverting or guiding structure of the particle trap. This means, for example, that a lower degree of diversion of the flow, with smaller diverting structures, is required if a sufficiently high porosity of the channel wall is ensured. In this case, in particular, regions with reduced pressures in adjoining flow paths are produced close to and downstream of a diverting structure, as seen in the flow direction, so that the particles are sucked into the porous material. If the particle trap is suitably adapted to the prevailing flow conditions of the exhaust gas flowing in, particularly effective separation of particles in combination with very low pressure losses is ensured.
With the objects of the invention in view, and in accordance with a practical implementation, the particle trap forms part of a honeycomb body with a multiplicity of flow channels. In this embodiment, the particle trap is particularly suitable for automotive engineering. In order to cover various (dynamic) load changes of the drive system of a motor vehicle, a conical system or an element in cone form is preferred. Such systems as described, for example, in International Publication No. WO 93/20339, have widening channels, so that particularly favorable conditions for trapping particles are established for any mass throughput at some point in the channels, if they are provided with appropriate diverting or turbulence-imparting structures.
In accordance with another mode of the invention, the separation of particles out of a fluid flow is perfected by regenerating the porous or highly porous channel wall continuously or discontinuously. In this case, various auxiliaries can be added in combination or individually. Auxiliaries are, on one hand, upstream catalytic converters which provide sufficient NO2 and/or upstream additions of additives which promote oxidation of the particles and therefore regeneration of the particle trap.
In addition, it is also possible for a NO2 accumulator to be flow-connected to the particle trap, for example an accumulator made from perovskite, such as BaSnO3 or another barium-tin perovskite, or a NO2 accumulator made from barium aluminate and/or an accumulator made from a barium zeolite.
In accordance with a further mode of the invention, the process is assisted by auxiliaries which are released as a function of the degree of occupation of the particle trap. This process can therefore be initiated in accordance with a pressure drop which is determined by the particle trap.
With the objects of the invention in view, there is additionally provided an exhaust system of an internal-combustion engine, in particular a diesel engine, in which the particle trap according to the invention is particularly advantageously used. It is especially advantageous for the particle trap to be used in combination with a soot filter. In this case, the soot filter will, of course, be considerably smaller than in exhaust systems without a particle trap.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a particle trap and a process for separating particles out of an exhaust gas flow and a honeycomb body and an exhaust system having a particle trap, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.